This invention relates to head fixation apparatus primarily but not exclusively for use in stereotactic procedures for diagnosis and treatment of the brain.
A number of different imaging techniques are available for diagnosis and monitoring of neurological conditions including computerised tomography, positron emmision tomography, magnetic resonance imaging, magnetic resonance spectroscopy, and digital subtraction angiography. It is often desirable that imaging be carried out periodically and for images from different types of apparatus to be integrated or correlated. A problem in each case arises in repeatably positioning the patient""s head in a precise location relative to each imaging apparatus and defining a co-ordinate system enabling different apparatus to be used to image the same image plane.
A number of stereotactic surgical techniques are based on imaged information and it is also a common requirement for stereotactic procedures such as biopsies to be carried out at the same location within a patient""s head on a number of separate occasions over a long term period of treatment. Examples of such surgical procedures would be the biopsy of a tumour and the draining of cysts.
It is known to provide head fixation apparatus comprising frame means attachable to stereotactic surgical and or imaging apparatus and fixation means for fixing the frame means onto a patient""s head in a substantially reproducible manner such that the location of anatomical features and their corresponding images can be spatially defined with reference to the frame means.
The co-ordination of stereotactic imaging and surgical procedures has for example been achieved by means of head fixation apparatus in which a frame means is a rigid metal frame fixed to a patient""s head so that a three dimensional coordinate system can be defined with reference to the frame. Biomedical images can then be characterised with reference to this coordinate system and the co-ordinates used as control parameters for stereotactic procedures. A guide for a surgical probe can for example be attached to the frame and used to guide the probe to co-ordinates within the brain which have been calculated from an image that may have been produced sometime earlier at a different location having the appropriate imaging facilities.
For satisfactory correlation to be achieved it is necessary that the fixation means by which the frame means is fixed to the patient""s head provides precise and repeatable location of the frame means with reference to the patient""s fixed anatomical features i.e. the skull bones.
It is known for the fixation means to comprise rods or screws which penetrate holes in the patient""s skull and which support a frame means in the form of a base ring. Imaging apparatus or stereotactic surgical instruments may then be connected to the base ring. A number of disadvantages are associated with this fixation means, primarily because of its invasive nature which requires sterility and anaesthesia and results in patient trauma. The reproducibility available with this technique is also limited because the bore holes become obstructed by debris and will heal over in time preventing long term relocations.
A number of non-invasive techniques have been proposed of which GB-2164856 (Laitinen) discloses fixation means for a reference frame comprising two ear plugs and a naison support. A disadvantage of this apparatus is that the ear plugs and naison support each contact relatively soft tissue so that to minimise movement of the frame means a degree of force needs to be applied to the ear plugs and naison support which is uncomfortable for the patient and prevents the apparatus being tolerated for more than about 20 minutes. This factor makes the apparatus unsuitable for the prolonged studies demanded in some imaging techniques such as positron emmission tomography. A further disadvantage is that the particular apparatus disclosed is insufficiently robust to serve as a secure base for surgical instruments.
It is also known to mould a helmet conforming to the shape of the top part of a patient""s head using either thermosetting material or fibreglass/resin techniques and to inset metallic supports into the helmet upon which to mount a frame. Such systems are described by T. Greitz et al in Neuroradiology 19, 1-6(1980).
This publication discloses the use of a dental mould in conjuction with a fibreglass bandage to provide a mouth piece receiving an impression of the patient""s upper incisor teeth.
A disadvantage of such skull cap techniques is their inherent lack of reproducibility due to the presence of soft tissue between the cap and the skull. The skin in this area is also not firmly fixed to the skull and tends to allow movement of the cap. In the case of a skull cap in which a mouth piece is provided having an impression of the upper incisors it is found that cap movement tends to be pivotal about the incisors. Consequently this technique has not been found to be practicable in general use.
It is also known to provide fixation apparatus where reference is made to the patient""s teeth as in the case of the Rand-Wells stereotactic device referred to in Radiology Volume 74 No. 1 pages 86 to 87 January 1960. Although not disclosed in this publication the Rand-Wells device in use includes a mouthpiece having a U-shaped tray into which dental cast material is received to make a cast of the patient""s upper teeth. Subsequent to alignment with the teeth three pointed screws are fastened to the skull through the skin such that a framework is rigidly and precisely fixed to the head. The Rand-Wells device has been used for procedures in which radio active seeds are implanted into the pituitary gland through the nasal cavities. The invasive nature of the screws leads to the disadvantages of other invasive techniques discussed above.
According to the present invention there is disclosed apparatus for use in stereotactic diagnosis and treatment comprising frame means, or frame assembly, and fixation means, or fixation member, for repeatably and reproducibly fixing the frame means to the patient""s head such that locations within the head are spatially definable with reference to the frame means, wherein the fixation means comprises a cast of the patient""s upper teeth and or palate, the fixation means further comprising support means for rigidly connecting the cast to the frame means and biassing means non-invasively engageable with the head and operable between the head and the frame means and or the fixation means to bias the cast into positive contact with the upper teeth and or palate.
Preferably the fixation means comprises a cast comprising a U-shaped portion indented with an impression of substantially all of the patient""s upper teeth if any.
An advantage of such an arrangement is that the teeth provide the most rigid point of non-invasive location on the skull and are able to withstand comfortably considerable firm pressure particularly when distributed over the complete set of teeth.
Advantageously the cast has a contact surface which in use engages in intimate contact the inner and inferior aspects of all of the patient""s upper teeth.
It is thereby possible to make the cast conform to the numerous contours provided by the patient""s teeth so that the cast locates in a unique position. This also avoids the problem of the undercut provided by the incisor teeth in that a cast which also fitted intimately over the entire inner and outer surface of these incisor teeth would not be removable without splitting the cast.
Preferably the contact surface further engages the outer aspect of the molar and premolar teeth.
Additional locating points are thereby established without preventing the cast from being removed since these teeth in general do not include an undercut.
The cast may include also a bridge portion conforming to the palate and forming a continuous bridge between the arms of the U-shaped portion.
The hard palate is also capable of comfortably taking high loads when evenly distributed and of giving reliable relocation characteristics because it is formed by a thin layer of a membrane (mucoperiosteum) which is firmly fixed to the underlying palate bone. This membrane is approximately one millimetre in thickness and is not only relatively incompressible compared with other skin areas but also does not tend to slide on the bone unlike skin covering the scalp.
The palate is dome like in shape and is therefore well suited for restraining the cast against movement in all directions other than downward.
The upper palate bone is also ridged and the membrane conforms to this ridging thereby providing additional means for positively locating the cast.
Such a bridge arrangement has the advantage of providing considerable strength to the cast since a bridge or dome like configuration has improved dimensional stability under compressive loading.
The bridge portion may be omitted where the patient""s upper teeth are such as to provide adequate seating of the cast in that, when biassed into contact with the teeth the cast is firmly located in a stable fixed position. The omission of the bridge under these conditions facilitates speech by the patient which would otherwise be impaired by restricted tongue movement.
Preferably the support means comprises a plate having a central portion connected to the front of the cast so as to be generally coplanar with the U-shaped portion and side portions which extend rearwardly inside of the patient""s mouth in supporting contact with the cast to at least as far as the location of the premolar teeth.
Preferably the plate includes side portions which extend rearwardly outside of the patient""s mouth to at least as far as the location of the premolar teeth.
Such an arrangement allows a load to be applied to the cast in a direction which is upwards with respect to the normal orientation of the skull with the load being directed in the region of the premolar teeth. This configuration has been found to be most comfortable and stable in use.
Advantageously the side portions extend at a downwardly inclined angle with respect to the central portion.
The plate is thereby formed in an arch configuration which is dimensionally stable under load.
The support means may include a tray connected to the plate and containing a layer of a material which is initially indentable so as to take an impression of the patient""s upper teeth and/or palate and which is subsequently cured to a rigid form retaining the impression formed therein.
Conveniently the apparatus includes demountable connecting means operable between the fixation means and the frame means.
It is then possible to use the same frame means with a plurality of different fixation means adapted to corresponding individual patients by including casts of their own teeth and or palate.
Conveniently such connecting means are provided by vertical location plates connected to the side portions of the support means and having screw fittings operable between the location plates and the frame means.
Advantageously the side portions are fixed to the location plates after an initial positioning procedure during which the frame means is supported at a desired position and the corresponding relative positions of the side portions and the location plates are recorded. The side portions may be fixed to the location plates by screw fittings or by adhesive bonding.
Such an arrangement enables the frame means to be positioned relative to the patient""s head in a manner which is adapted to the patient""s individual physical dimensions and the position and nature of the abnormality requiring investigation and treatment. The frame means can thereby be used on any number of different patients without modification and those parts of the apparatus which are specific to a particular patient are contained in a single unit which can be conveniently stored.
Preferably the apparatus includes positioning arc means detachably connectable with the frame means and defining a working space relative to the frame means which is equal to or less than the corresponding working space required by stereotactic instruments or the like which are to be subsequently fitted to the frame means in use whereby the initial positioning procedure carried out with the arc so connected ensures the frame means is positioned relative to the head so as to maintain working clearance between the head and any such instrument.
Such an arrangement avoids the need to carry out the initial positioning procedure whilst the patient""s head is positioned within the stereotactic instrument to be used. The arc means may also be formed from a lightweight material which enables the apparatus to be easily manoeuvred during the initial positioning procedure.
Conveniently the biassing means comprises straps connected to the frame means and extending over the top of the patient""s head.
Preferably the biassing means further includes a headrest connected to the frame means and engageable with the lower rear portion of the head to resist forward tilting on the frame means.
The biassing means may then operate by tightening the straps so that the frame means is biassed upwardly with a thrust directed generally at its centre. Since the cast is located at the front of the skull there is a tendency for forward tilting to arise since the downward reaction on the frame means from the patient""s teeth and palate is located at the front of the frame. By including the headrest a downward reaction at the rear of the frame means is provided which enables the frame means to be stable under loaded conditions.
Advantageously the headrest includes a cushion comprising a flexible casing containing a granular material and having means for evacuating air from the casing. Such a cushion can be intimately moulded to the shape of the head and becomes a ridged support once the air has been evacuated from the casing. Alternatively the cushion may be inflatable with air or liquid to assist distribution of loading on the head.
Preferably the apparatus includes ear supports comprising ear plugs detachably connectable by adjustable supports to the frame means. Such ear plugs are useful in fitting the apparatus and assist in maintaining the correct alignment of the frame means prior to tightening of the staps and positioning of the headrest.
Alternatively the headrest may comprise a rigid pad moulded to the occipital region of the patient""s head and calibrated adjustment means operable between the pad and the frame means for repeatably positioning the pad at co-ordinates specific to each patient. The use of ear plugs referred to above would in this arrangement then be unnecessary.
Conveniently the frame means comprises a U-shaped frame member and a U-shaped bracket upon which the headrest is mounted and being connectable with the frame member to form a closed annular frame and adjustment means facilitating adjustment of the relative positions of the frame member and bracket to thereby adjust the position of the headrest relative to the head.
Preferably the frame means and fixation means comprise non metallic and insulating materials.
Such materials do not introduce artefacts under most imaging systems including magnetic resonance imaging.
Such apparatus may nevertheless include fiducial features which are specifically selected from visible materials under the required imaging system for example where it is required to calibrate a system prior to or during use.
Preferably the apparatus further comprises a frame holder detachably connectable to the frame means and having means for connection to biomedical scanning apparatus and having means for adjusting the position of the frame means relative to the frame holder in a plane which is parallel with the plane imaged by the scanning apparatus.
Such an arrangement ensures that all scanned images obtained using such scanning apparatus are taken in parallel planes. In conventional scanning apparatus a series of images are taken at planes which are parallel to each other and spaced apart by regular intervals and it is important that no tilting of the patient""s head occurs between the taking of subsequent images.
A particular advantage of the present invention when used in this way is that the frame holder may be attached to a number of different types of biomedical scanning apparatus thereby allowing subsequent scans using different apparatus to be taken at the same plane relative the patient""s head. Furthermore by taking a series of scans which are spaced apart by the same distance for each apparatus it is possible to integrate the biomedical images obtained by different scanning apparatus using appropriate computer techniques.
This is particularly useful for example when using positron emission tomography which does not produce images of the patient""s skull. It is possible in accordance with the present invention to correlate and integrate such images with computerised tomography scans which image the patient""s skull so that after subsequent processing the features imaged on the positron emission tomography scan can be visually related to the skull.
A further advantage of the use of such a frame holder is to enable the patient""s head to be repeatably positioned within apparatus for irradiating a localised area within the skull.